MGa2S4: Eu materials, where M comprises at least one of Mg, Ca, Sr and Ba, show green color emission around 535 nm, and provide high lumen efficacy of LED lighting. However, the narrow band green phosphors with general composition MGa2S4: Eu are hygroscopic, and exhibit rapid deterioration of photoluminescence due to exposure to moisture (water vapor), oxygen and/or heat. There is a need for narrow band green phosphors with general composition MGa2S4: Eu, with coatings which are effective at protecting the phosphor particles from moisture and oxygen and enable a commercially useful phosphor.
Furthermore, there is a need for new compositions of narrow band green phosphors based on the phosphor with general composition MA2S4: Eu, wherein: M comprises at least one of Mg, Ca, Sr and Ba, A comprises at least one of Ga, Al, In, La and Y, providing improved lumen maintenance and stable chromaticity over the lifetime of a light emitting device.